The invention relates to a method of manufacturing powdered barium titanate, in which titanium dioxide is made to react with barium hydroxide at a temperature in the range from 60.degree. to 95.degree. C. while excluding CO.sub.2.
Ceramic powders having the Perowskite structure such as, for example, barium titanate powders, are an important starting material for the manufacture of widely differing electronic components such as, for example, multilayer capacitors or non-linear resistors having a positive temperature characteristic. The demand for ever improving electronic components as well as an improved process control in the manufacture of said components imposes high requirements on the starting materials. In particular, fine powder particles having a round shape and a narrow-range particle size distribution are in demand. Moreover, the starting materials must be of a high purity. Besides, if large-scale manufacturing is to be economical, the methods used for the manufacture of the BaTiO.sub.3 powders must be inexpensive and must be carried out in a short period of time.
Barium titanate powders are produced by widely differing methods which, however, all have certain disadvantages, in particular, as regards purity, particle size and particle size distribution.
Barium titanate powders can for example be manufactured according to the so-called mixed-oxide method. In this method, barium carbonate and titanium dioxide are mixed in a grinding process and converted into barium-titanate at high temperatures of about 1000.degree. C. A disadvantage is, that in the necessary grinding processes the relevant powder is contaminated in an undesired manner by abrasion. A further disadvantage is that high calcining temperatures are required, causing large aggregated BaTiO.sub.3 powder particles having an irregular shape and a wide-range particle size distribution to be formed. The resultant particles must be reduced in size after the calcining process in a grinding process in which, however, particle sizes &lt;1 .mu.m cannot be attained. Moreover, the calcining process involves a high energy consumption.
Further, it is known from, for example, J. Am. Cer. Soc. 48 (1965), p. 644 ff. to manufacture barium-titanate powder by pyrolysis of precipitates, for example barium titanyl oxalates. Due to the precipitation this method leads to a powder of a higher purity than that obtained with the mixed-oxide method, however, it also requires reaction temperatures of approximately 1000.degree. C. The powder aggregates formed in this process must also be reduced in size by a grinding process.
Fine highly pure barium-titanate powder having a particle diameter in the range from 0.005 to 0.03 .mu.m can be manufactured according to known methods by precipitation at low temperatures in the range from 20.degree. to 90.degree. C.: These metals involve
1. Hydrolysis of barium titanium alcoholates with H.sub.2 O (J. Am. Cer. Soc. 54 (1971), pp. 548-553), or
2. Hydrolysis of titanium alcoholates in an aqueous Ba(OH).sub.2 solution (J. Am. Cer. Soc. 49 (1966), pp.291-295).
Both of these known methods are very costly and time-consuming. To avoid decomposition of the alcoholates to BaCO.sub.3, TiO.sub.2 or Ba(OH).sub.2 the operation must be carried out in an atmosphere which is free of H.sub.2 O, CO.sub.2 and O.sub.2.
If the reaction temperature, the concentration and the precipitation rate is not carefully controlled, undesired deviations from the morphology and the stoichiometry of the barium-titanate powder take place.
Barium-titanate powder can further be obtained by a hydrothermal reaction. By means of this method which is known from, for example, German patent application No. DE 3526674 fine barium titanate powder can be manufactured by using hydroxides, halogenides or alcoholates at temperatures in the range from 150.degree. to 200.degree. C. and pressures between 5 and 15 bar. The cost of this known method is relatively high because the reaction must take place in autoclaves and certain safety precautions have to be taken. A further disadvantage may be that the barium-titanate powder can only be produced batchwise instead of in a continuous process.
From EP-A 141551 a method of manufacturing alkaline earth metal titanate powders is known, in which titanium oxide is made to react with barium hydroxide at a reaction temperature in the range of from 60.degree. to 110.degree. C. Preferably, orthotitanium acid having a high water content is used as titanium oxide, this acid is obtained by a reaction of a titanium salt such as a chloride, sulfate or oxalate with alkalis. Orthotitanium acid contains at least 2 mol. of water. However, metatitanium acid having a water content of 1 mol. or titanium dioxide, obtained by heating orthotitanium acid, may alternatively be used.
A disadvantage of this method is that the starting compounds (chlorides, sulphates, oxalates, alkalis) used contaminate the end product to be manufactured. Perovskite-phase powders which are to be used for the manufacture of electronic components must be very pure. If orthotitanium acid is heated to produce titanium dioxide, there is the problem that the titanium dioxide crystallizes and, hence, exhibits a reduced reactivity in subsequent process steps. Moreover, a further problem is that, due to the heating of the orthotitanium acid and the consequential crystallization process, undesired impurities such as alkali ions, chloride ions, sulphate ions, oxalate ions may be introduced into the crystal lattice, which impurities cannot be removed in subsequent purification processes.